Automatic reclosing (ANSI 79) — why an overhead MV line automatically closes back in after tripping

The [guide on directional overcurrent protection (ANSI 67)](/guides/nen-3140/richtingsafhankelijke-overstroombeveiliging-67-richtingsrelais) covers how a protection recognizes that a fault exists and in which direction it lies. This article covers what often happens automatically on an overhead medium-voltage line after a breaker has tripped on such a fault: automatic reclosing, designated by ANSI code 79 and in practice often carried out by a recloser.

Why reclosing makes sense: most faults are transient

On an overhead line, a substantial share of faults is caused by events that resolve themselves quickly: a lightning strike causing a brief flashover, a branch briefly touching the line, or a bird causing an arc flashover and then falling away. For this kind of transient fault, the ionized air of the arc has recovered sufficiently after a brief, de-energized period for the line to be safely re-energized without damage. A breaker that stays permanently open after every trip would cause an unnecessarily long supply interruption for this kind of short-lived, self-clearing fault. Automatic reclosing exploits this distinction: after a trip, the line is automatically re-energized after a set dead time, on the assumption that the majority of faults have by then cleared on their own.

Dead time: long enough to let the arc extinguish

The dead time — the time the line stays de-energized between tripping and reclosing — must be long enough to let the ionized air around the fault location de-ionize, so that the arc does not immediately re-strike as soon as the line is re-energized. For overhead lines, a common dead time for the first, fast reclosing attempt is on the order of a few tenths of a second up to about one second. Too short a dead time increases the risk that the arc re-strikes; an unnecessarily long dead time extends the interruption for connected customers without additional benefit.

Reclosing scheme: from fast to slow, multiple attempts

A typical reclosing scheme combines multiple attempts with progressively longer dead times:

A first, fast reclosing attempt follows shortly after tripping (on the order of a few power-cycle periods up to a fraction of a second), intended to clear the shortest, most immediately de-ionizing faults with a barely noticeable voltage interruption.
If the fault recurs after this fast attempt as well, the protection typically switches to one or more slower reclosing attempts, with a dead time of a few seconds up to roughly ten seconds, to give a slightly more persistent but still transient fault the chance to clear itself.
If the fault is still present after all set attempts, it is concluded to be a permanent fault (for example a broken conductor or a sustained short circuit), and the breaker stays open permanently — often via a lockout relay (ANSI 86) that requires a manual reset before the line can be closed again.

Why not keep reclosing indefinitely

The number of reclosing attempts is deliberately limited: reclosing repeatedly onto a permanent fault would repeatedly expose the fault location to the full short-circuit current, could cause additional damage (for example to an already-damaged conductor or a faulty joint), and would pose an increased safety risk to any person or animal that happens to be near the fault location. A limited number of attempts gives transient faults a fair chance to clear on their own, without repeatedly "trying" a permanent fault unnecessarily.

Note: the exact number of reclosing attempts, the dead times, and the coordination with other protection functions follow from the grid configuration and the grid operator's requirements; this article covers the principle, not a ready-made setting table for every line.

Practical relevance

When working on or near an overhead medium-voltage line fed through a recloser, it is essential to disable the reclosing function beforehand — without this measure, the line could unexpectedly be automatically re-energized during the work after a trip, posing a direct hazard to anyone working on the line at that moment.

Common mistakes

Not disabling the reclosing function before working on the line — this is a direct safety risk to people working on an apparently de-energized line.
Applying the same reclosing setting to an overhead line as to an underground cable connection — a fault on a cable is almost always permanent, so automatic reclosing there is typically pointless and can actually increase the damage.
Setting too short a dead time to minimize the interruption — this increases the risk that the arc re-strikes before the air around the fault location has sufficiently de-ionized.
Allowing an unlimited number of reclosing attempts — this repeatedly exposes a permanent fault to the full short-circuit current and increases the risk of additional damage.